It's been a while since 'the dress', we all know the one by now, appeared online. You may have already seen my Facebook post explaining why some people saw white and gold and others saw black and blue but I thought it would be a great idea to explain things in a bit more detail.

If you really want to break the internet you don't need a Kardashian, you need a dress that looks different colours to different people. #thedress #blueandblack #whiteandgold

The most important thing to know is that you aren't crazy and that seeing either colour combination is normal. As with so many things in life what is important is context. A perfectly innocent phrase heard out of context may sound shockingly inappropriate. A perfectly blue and black dress in the wrong lighting may suddenly seem to be white and gold to certain people. What makes this situation interesting and unique is that different people seeing the same dress under the same conditions see different things. It's the fault of the lighting in the picture, NOT the lighting around you when you look at the picture.

The frilly part of the dress, according to most reliable sources, is black and the rest of the dress is really blue. So how can someone possibly see the black part of the dress as yellow and the blue part of the dress as white? To help this make sense all we need is a little help from the CIE colour chart below.

This is the CIE colour wheel. It is ridiculously helpful at making confusing colour problems make sense. Add or subtract certain amounts of light away and you move from one end of the scale to the other. Blue, as seen here in the bottom left, can become white and eventually yellow if you add more and more yellow light.

The key to the dress is the surrounding lighting. From the chart above it's not hard to see that if we add yellow light to the colour blue it slowly will shift from blue to white. Black is the absence of any reflected light. If more yellow is added to the colour black it will change to yellow or in this case, gold. So if we add more yellow lighting to a picture of a blue and black dress it makes a lot of sense that it would appear to be white and gold instead (to be clear, this is only true of reflected light. If you add lots of yellow paint to blue or black paint this won't work and you'll just end up with a mess). What makes the dress really interesting is that some people see it as blue and black and some people see it as white and gold while looking at the exact same picture at the exact same time. Why is this happening?

Whenever you look at any colour in any lighting condition your brain has to edit out the surrounding lighting so things don't look like totally different colours at different times of the day and different lighting conditions. In most situations and looking at most pictures we all subtract out the same colours or add in the same colours. In the case of the dress a certain number of people's brains subtract out the yellowish lighting so the dress looks blue and black. Some people's brains add in more yellow making the dress look a vibrant white and gold. The dress may really, truly be blue and black but each of us will perceive the dress differently depending on whether we subtract out the yellow light or add in more.

So, in the end, you aren't colour blind and you aren't going crazy. It's totally the fault of your visual cortex and some higher end perceptual calculations that you can't control. I hope this has helped make the dress make way more sense.

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